If you’re a teenager, whether you’re an adult in the eyes of the law depends on the state you live in. Slowly, that’s starting to change.

Today, most states, including Washington, D.C., treat adolescents as juveniles through the age of 17. Nine states still have a lower bar, at age 16, apart from New York and North Carolina, where the age is 15.

Over the past decade, a bipartisan movement of legislators and advocates has been pushing to raise the age of criminal responsibility for juveniles. They are bolstered by neuroscience that shows the still developing adolescent brain is more prone to taking risks and less capable of making decisions. And they are motivated by a pragmatic desire both to reduce recidivism and cut incarceration costs.

In the last seven years, Illinois, Connecticut, Massachusetts, New Hampshire and Mississippi have raised the age when adolescents can be tried as adults to 18. Of the nine states that currently have a lower bar, five have passed or are currently considering legislation to raise it.

Last week, South Carolina’s legislature unanimously voted to raise the age when adolescents can be tried as adults to 18, sending a bill to Republican Gov. Nikki Haley, which she is expected to sign. Louisiana’s Republican-controlled legislature passed a similar bill on Thursday, which the Democratic governor, John Bel Edwards, has said he would sign. Michigan and New York are both considering similar legislation.

“I think we’ve won the moral and research argument that this is a failed public policy, to have kids under 18 treated uniformly as adults,” said Marcy Mistrett, chief executive of the Campaign for Youth Justice, a Washington, D.C.-based advocacy group. “And now it’s really about how we’re going to fund it, how we’re going to implement it.”

Through the 1990s, fearing a wave of juvenile crime, the vast majority of states made it easier to prosecute adolescents as adults. States increased their use of waivers, which allowed prosecutors to send juveniles to the adult system for committing serious violent crimes. And some raised the age of criminal responsibility for every offense.

But the crime wave never came, and juvenile arrests instead began a steady decline. Beginning in 2005, the Supreme Court issued a string of decisions on youth imprisonment, citing new research on brain development to determine that juveniles should be treated differently than adults. That year, in Roper v. Simmons, the court banned the execution of youth offenders. In 2010, in Graham v. Florida, the court ruled against mandatory life sentences for juveniles convicted of crimes other than homicide. Two years later, in Miller v. Alabama, the court expanded the ban on mandatory life sentences for juveniles convicted of any crime.

Now, states are starting to shape their policies to reflect this. In Louisiana, support for the bill has been driven in part by a study by the state’s public health department, which argued that 17-year-olds should be kept in the juvenile system because they are still developing and have a greater potential for rehabilitation.

“In the state of Louisiana, a 17-year-old can’t sign a contract, they can’t smoke, they can’t vote,” said Sen. Jean-Paul Morrell, a Democrat who sponsored his state’s bill. “All these things we limit their ability to do because we say, ‘You’re a child.’ However, if you swipe a Coke from a department store, you can go to jail for 10 years. It didn’t make sense.”

Louisiana has also passed a bill requiring the state to ensure that adolescents in juvenile facilities continue to receive education and that their cases are periodically evaluated while in detention.

Evidence also suggests that locking up kids in adult prison generally leads to more crime. Studies that have compared teens sent to criminal court with those tried for similar crimes in the juvenile system have found that those in the adult system were more likely to reoffend at a higher rate and more quickly than those in juvenile court. A 2007 analysis of studies by a Centers for Disease Control and Prevention task force, for example, found a 34 percent increase in violent or general crime for adolescents who were sent to the adult system, compared to those who remained in the juvenile system.

And a 2010 Justice Department study found that transferring kids to the adult system doesn’t deter crime.

Unlike juvenile facilities, adult prisons aren’t focused on rehabilitation. Young people in adult prisons may not have access to age-specific programming or education, making it more difficult for them to re-integrate into society upon their release. Youths can also suffer long-term damage in adult detention, where they’re at risk for sexual and physical violence, and have a much higher risk of suicide.

As conservatives who see raising the age as a way to cut costs will note, it’s also expensive to incarcerate young people — as much as $400 per day, according to a study by the Justice Policy Institute, compared to less than $75 per day for community programs. And because adolescents face heightened risks of abuse in adult prisons, states must also make additional accommodations to protect them.

Under federal law, young people under age 18 in adult facilities must be housed separately from adult populations to protect them from physical and sexual abuse. That creates a “third tier,” of inmates who aren’t considered juveniles by the state, but can’t be treated like other adults and require specialized care — which drives up costs, said Derek Cohen, deputy director of the Texas Public Policy Foundation’s Center for Effective Justice and Right on Crime, a conservative policy think tank.

But there are still limits to the broad consensus, which are rooted in some lawmakers who remain concerned about appearing too soft on juvenile offenders.

This week, advocates in New York said that public safety concerns from some legislators had scuttled chances of a bill passing this session, which ends June 16. “It just seems like we haven’t been able to get it across the finish line,” said Stephanie Gendell of the Citizens’ Committee for Children, an advocacy group. “There’s a belief that [raising the age] is soft on crime,” she added.

And attempts in three states — Connecticut, Vermont and Illinois — to extend the juvenile ceiling even further, to all those under 21, have failed so far. In Connecticut, Democratic Gov. Dan Malloy, who had been advocating for the change, dropped the language from a reform bill on Wednesday amid concerns about public safety from the Republican leadership, though advocates said they’ll continue to press for the issue going forward.

Despite such resistance, even traditionally tough-on-crime states have begun to move towards alternatives to incarceration for juveniles. In Michigan, for example, a bill to raise the age of responsibility to 18 is part of a package that would establish more funding for social services for youths, and for those in detention, ensure they have age-appropriate programming and maintain connections with their families.

“Systems are changing,” said Kristen Staley, deputy director of the Michigan Council on Crime and Delinquency. “We’re seeing fewer and fewer young people being held in locked facilities and a huge growth in community efforts.”

This story has been updated.

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